Stuck on Foreign Keys

Hi

I have a question regarding foreign keys, I just cannot get it to create
them for me....I must be doing something wrong but I have no idea what
that might be :)

I have a table "clients"

clientID (primary)
ticode
Firstname
SecondName

I have a second table "titles"

ticode (primary)
Title

I am trying to create a foreign key on TIcode "clients" table as in below,

ALTER TABLE clients ADD CONSTRAINT the_title FOREIGN KEY (ticode)
REFERENCES titles (ticode) ;

I keep getting this error

ERROR:  insert or update on table "clients" violates foreign key
constraint "the_title"
DETAIL:  Key (ticode)=( ) is not present in table "titles".

Sorry, I have no idea where I  am going wrong...Any help would be great

TIA

C

.